1. Field of the Invention
The present invention relates generally to an image projection system implemented with a mirror device commonly known as a “digital micromirror device” or “micromirror device”. More particularly, this invention relates to an image projection system implemented with a mirror device that includes micromirrors having a specific range of natural oscillation frequency and operated with more than three controllable states wherein one of the controllable states is related to the natural oscillation frequency of the micromirrors.
2. Description of the Related Art
Even though there have been significant advances made in recent years in the technology of implementing electromechanical micromirror devices as spatial light modulators, there are still limitations and difficulties when these are employed to provide high quality image displays. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. FIG. 1A refers to a digital video system 1, disclosed in a relevant U.S. Pat. No. 5,214,420, that includes a display screen 2. A light source 10 is used to generate light energy for the ultimate illumination of display screen 2. Light 9 generated is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13 and 14 form a beam columnator, which operates to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer 19 through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. As shown in FIG. 1B, the SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5, where it is enlarged or spread along path 4 to impinge onto the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected towards display screen 2 and hence pixel 3 remains dark.
The on-and-off states of the micromirror control scheme, as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display systems, impose a limitation on the quality of the display. Specifically, in a conventional configuration of the control circuit, the gray scale (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the on-off states implemented in the conventional systems, there is no way to provide a shorter pulse width than LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, FIG. 1C exemplifies a conventional circuit diagram of control circuit for a micromirror, according to the U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads of the memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a word-line. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The dual-state switching, as illustrated by the control circuit, controls the micromirrors to position either at an ON or an OFF orientation, as that shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is in turned controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness for each of the four bits, where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales is a brightness represented by a “least significant bit” that maintains the micromirror at an ON position.
When adjacent image pixels are shown with a great degree of difference in the gray scales, due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are especially pronounced in the bright areas of display, where there are “bigger gaps” between gray scales of adjacent image pixels. For example, it can be observed in an image of a female model that there are artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated by technical limitations in that the digitally controlled display does not provide sufficient gray scales. Thus, in the bright areas of the display, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully on and fully off position, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of a display, the speed of the micromirror must be increased such that the digital control signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a stronger hinge is necessary for the micromirror to sustain the required number of operational cycles for a designated lifetime of operation, In order to drive micromirrors supported on a stronger hinge, a higher voltage is required. In this case, the voltage may exceed twenty volts and may even be as high as thirty volts. Micromirrors manufactured by applying the CMOS technologies would probably not be suitable for operation at this higher range of voltages, and therefore, DMOS micromirror devices may be required. In order to achieve a higher degree of gray scale control, more complicated manufacturing processes and larger device areas are necessary when DMOS micromirrors are implemented. Conventional modes of micromirror control are therefore facing a technical challenge in that the gray scale accuracy has to be sacrificed for the benefits of smaller and more cost effective micromirror display, due to the operational voltage limitations.
There are many patents related to light intensity control. These Patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different shapes of light sources. These Patents includes U.S. Pat. Nos. 5,442,414 and 6,036,318 and Application 20030147052. U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents and patent application do not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation including U.S. Pat. Nos. 20,25,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed or provided direct resolution for a person of ordinary skill in the art to overcome the limitations and difficulties discussed above.
Therefore, a need still exists in the art of image display systems, applying digital control of a micromirror array as a spatial light modulator, for new and improved systems such that the difficulties and limitations discussed above can be resolved.
Incidentally, an address electrode for driving a mirror is placed under the mirror. The reason is that the mirror and address electrode need to be placed as closely to each other as possible in order to effectively generate a sufficient magnitude of a coulomb force for driving the mirror. The coulomb force for driving the mirror is inversely proportional to the second power of the distance between the electrode and mirror. Further, the coulomb force is also dependent on the area size of the address electrode, that is, the coulomb force increases with the area size of the address electrode, and an address electrode with a sufficient area size needs to be placed under the mirror.
A miniaturization of a mirror device is naturally accompanied by a reduction in the space for placing an address electrode. In addition, a stopper is placed under the mirror, separate from the address electrode, to regulate the deflection angle of a mirror. The stopper abuts the mirror when the mirror is fully deflected. In a situation in which the mirror device is miniaturized, decreasing the space available for placing the address electrode, the conventional method of configuring of the address electrode and the stopper will be faced with a technical problem in that the space for placing the address electrode is further reduced, making it very difficult to obtain a sufficient magnitude of the coulomb force.
FIG. 2 is a cross sectional view for showing the structure of a mirror device for controlling a mirror deflection angle in the conventional mirror device, as disclosed in U.S. Pat. No. 5,583,688. This mirror device comprises a landing yoke 310, which is connected to a mirror 300. The yoke 310 deflects with the mirror 300. The yoke 310 includes a tip 312 formed in a part of the landing yoke 310. The tip 312 contacts a metallic layer, which is formed separately from the address electrode 314 to stop the mirror before the mirror 300 deflects to an angular position to come into contact with the address electrode 314, thereby regulating the deflection angle of the mirror 300. In such a configuration, the landing yoke and tip occupy part of the space available for placing an electrode, making it difficult to increase the size of the address electrode.
FIG. 3 shows the structure for regulating a mirror deflection angle in the conventional mirror device, as disclosed in US Patent Application 20060152690. Although this patent application discloses a structure that has eliminated the landing yoke, however, the mirror device still has a tip as a separate component for determining the deflection angle of the mirror. The tip functioning as a stopper is disposed in the space that would be available for placing an address electrode. In a mirror device with configuration shown in FIG. 3, it would be difficult to increase the size of the address electrode.
FIG. 4 shows cross sectional views of a mirror to illustrate the structure for regulating a mirror deflection angle in the conventional mirror device, as disclosed in U.S. Pat. No. 6,198,180. In the mirror device disclosed by the patent, the configuration includes a stop post, which is separate from a capacitor panel to define the maximum deflection angle of the mirror. Therefore, the electrode size is still limited by the extra space occupied by the capacitor stop post and the capacitor panel.
FIG. 5 shows a cross section view of a mirror device for illustrating the structure for regulating a mirror deflection angle in the conventional mirror device, as disclosed in U.S. Pat. No. 6,992,810. The mirror device comprises a mechanical stop element, which regulates the deflection angle of a mirror, directly under the mirror. The mechanical stop element abuts on a landing electrode that is maintained at the same potential as the mirror. This disclosure also makes it difficult to increase the electrode size.
In order to provide mirror device by implementing the conventional pulse width modulator (PWM) to generate images with higher levels of gray scale, a higher drive voltage is required. A higher drive voltage is necessary due the requirement to deflect the mirrors supported on hinges that have a higher elasticity constant for mirrors oscillating at a higher speed in order to achieve higher levels of gray scale. For these reasons, there are still technical difficulties and limitation exist to further miniaturize the mirror device while providing improved quality of display images with higher resolution of gray scales by applying conventional mirror configurations and control techniques.